There was an accidental siren activation last night in the southwestern/western parts of St. Louis County (Missouri). According to local TV news reports, the "only way" that the sirens could have been activated is if someone somehow got a hold of the activation codes. So the blame is being placed on a hacker. Areas affected were Fenton, Chesterfield and Ballwin.
BTW, sirens involved would be Federal 2T22's, Whelen WPS-3016's and 4003's, and possibly some Whelen 2000's.